Signal quality determining device and method

ABSTRACT

A device for determining the quality of an output signal to be generated by a signal processing circuit with respect to a reference signal is provided with a first series circuit for receiving the output signal and with a second series circuit for receiving the reference signal and generates an objective quality signal by a combining circuit coupled to the two series circuits. Correlation between the objective quality signal and a subjective quality signal, to be assessed by human observers, can be considerably improved by coupling a converting arrangement to a series circuit for converting at least two signal parameters into a third signal parameter, and by coupling a discounting arrangement to the converter arrangement for discounting the third signal parameter at the combining circuit.

A. BACKGROUND OF THE INVENTION

The invention relates to a device for determining the quality of anoutput signal to be generated by a signal processing circuit withrespect to a reference signal, which device is provided with a firstseries circuit having a first input for receiving the output signal andis provided with a second series circuit having a second input forreceiving the reference signal and is provided with a combining circuit,coupled to a first output of the first series circuit and to a secondoutput of the second series circuit, for generating a quality signal,which first series circuit is provided with

a first signal processing arrangement, coupled to the first input of thefirst series circuit, for generating a first signal parameter as afunction of time and frequency, and

a first compressing arrangement, coupled to the first signal processingarrangement, for compressing a first signal parameter and for generatinga first compressed signal parameter, which second series circuit isprovided with

a second compressing arrangement, coupled to the second input, forgenerating a second compressed signal parameter, which combining circuitis provided with

a differential arrangement, coupled to the two compressing arrangements,for determining a differential signal on the basis of the compressedsignal parameters, and

an integrating arrangement, coupled to the differential arrangement, forgenerating the quality signal by integrating the differential signalwith respect to time and frequency.

Such a device is disclosed in the first reference: J. Audio Eng. Soc.,Vol. 40, No. 12, December 1992, in particular "A Perceptual AudioQuality Measure Based on a Psychoacoustic Sound Representation" by JohnG. Beerends and Jan A. Stemerdink, pages 963-978, more particularly FIG.7. The device described therein determines the quality of an outputsignal to be generated by a signal processing circuit, such as, forexample, a coder/decoder, or codec, with respect to a reference signal.The reference signal is, for example, an input signal to be presented tothe signal processing circuit, although the possibilities also includeusing, as reference signal, a pre-calculated ideal version of the outputsignal. The first signal parameter is generated as a function of timeand frequency by means of the first signal processing arrangement,associated with the first series circuit, in response to the outputsignal, after which the first signal parameter is compressed by means ofthe first compressing arrangement associated with the first seriescircuit. In this connection, intermediate operational processing of saidfirst signal parameter should not be ruled out at all. The second signalparameter is compressed by means of the second compressing arrangementassociated with the second series circuit in response to the referencesignal. In this connection, too, further operational processing of saidsecond signal parameter should not be ruled out at all. Of bothcompressed signal parameters, the differential signal is determined bymeans of the differential arrangement associated with the combiningcircuit, after which the quality signal is generated by integrating thedifferential signal with respect to time and frequency by means of theintegrating arrangement associated with the combining circuit.

Such a device has, inter alia, the disadvantage that the objectivequality signal to be assessed by means of said device and a subjectivequality signal to be assessed by human observers have a poorcorrelation.

B. SUMMARY OF THE INVENTION

The object of the invention is, inter alia, to provide a device of thetype mentioned in the preamble, wherein the objective quality signalwhich is to be assessed by means of the device and a subjective qualitysignal, which is to be assessed by human observers have an improvedbetter correlation.

For this purpose, the device according to the invention has thecharacteristic that the device comprises

a converting arrangement coupled to at least one series circuit forconverting at least two signal parameters into a third signal parameter,and

a discounting arrangement coupled to the converting arrangement fordiscounting the third signal parameter at the integrating arrangement.

As a result of providing the device with the converting arrangement andthe discounting arrangement, the complexity of the reference signal oroutput signal can be used to adjust the quality signal. Due to theconverting and discounting, a good correlation is obtained between theobjective quality signal to be assessed by means of said device and asubjective quality signal to be assessed by human observers.

The invention is based, inter alia, on the insight that the poorcorrelation between objective quality signals to be assessed by means ofknown devices and subjective quality signals to be assessed by humanobservers is the consequence, inter alia, of the fact that certaindistortions are found to be more objectionable by human observers thanother distortions, which poor correlation is improved by using the twocompressing arrangements, and is furthermore based, inter alia, on theinsight that distortions in a less complex signal are found to be moreobjectionable than distortions in a more complex signal.

The problem of the poor correlation is thus solved by an improvedfunctioning of the device as a result of providing the device with theconverting arrangement and the discounting arrangement.

A first embodiment of the device according to the invention has thecharacteristic that the converting arrangement converts at least asignal parameter at a first timepoint and at a first frequency andanother signal parameter at a second timepoint and at the firstfrequency into a fourth signal parameter at the first frequency andconverts a further signal parameter at a first timepoint and at a secondfrequency and another further signal parameter at a second timepoint andat the second frequency into a further fourth signal parameter at thesecond frequency, the discounting arrangement being situated between thedifferential arrangement and the integrating arrangement, and the thirdsignal parameter comprising the fourth signal parameter and the furtherfourth signal parameter.

In this case the adjustment is done before the differential signal isintegrated with respect to time and frequency.

A second embodiment of the device according to the invention has thecharacteristic that the converting arrangement converts at least asignal parameter at a first timepoint and at a first frequency andanother signal parameter at the first timepoint and at a secondfrequency into the third signal parameter at the first timepoint, thediscounting arrangement being situated inside the integratingarrangement for discounting the third signal parameter after thedifferential signal being integrated with respect to frequency andbefore the differential signal is integrated with respect to time.

A third embodiment of the device according to the invention has thecharacteristic that the second series circuit is furthermore providedwith

a second signal processing arrangement, coupled to the second input, forgenerating a second signal parameter as a function of both time andfrequency, the second compressing arrangement being coupled to thesecond signal processing arrangement in order to compress the secondsignal parameter.

If the second series circuit is furthermore provided with the secondsignal processing arrangement, the second signal parameter is generatedas a function of both time and frequency. In this case, the input signalto be presented to the signal processing circuit, such as, for example,a coder/decoder, or codec, whose quality is to be determined, is used asthe reference signal, in contrast to when a second signal processingarrangement is not used, in which case a pre-calculated ideal version ofthe output signal should be used as the reference signal.

A fourth embodiment of the device according to the invention has thecharacteristic that a signal processing arrangement is provided with

a multiplying arrangement for multiplying in the time domain a signal tobe fed to an input of the signal processing arrangement by a windowfunction, and

a transforming arrangement, coupled to the multiplying arrangement, fortransforming a signal originating from the multiplying arrangement tothe frequency domain, which transforming arrangement generates, afterdetermining an absolute value, a signal parameter as a function of timeand frequency.

In this connection, the signal parameter is generated as a function oftime and frequency by the first and/or second signal processingarrangement as a result of using the multiplying arrangement and thetransforming arrangement, which transforming arrangement also performs,for example, an absolute-value determination.

A fifth embodiment of the device according to the invention has thecharacteristic that a signal processing arrangement is provided with

a subband filtering arrangement for filtering a signal to be fed to aninput of the signal processing arrangement, which subband filteringarrangement generates, after determining an absolute value, a signalparameter as a function of time and frequency.

In this connection, the signal parameter is generated as a function oftime and frequency by the first and/or second signal processingarrangement as a result of using the subband filtering arrangement whichalso performs, for example, the absolute-value determination.

A sixth embodiment of the device according to the invention has thecharacteristic that the signal processing arrangement is furthermoreprovided with

a converting arrangement for converting a signal parameter representedby means of a time spectrum and a frequency spectrum to a signalparameter represented by means of a time spectrum and a Bark spectrum.

In this connection, the signal parameter generated by the first and/orsecond signal processing arrangement and represented by means of a timespectrum and a frequency spectrum is converted into a signal parameterrepresented by means of a time spectrum and a Bark spectrum by using theconverting arrangement.

The invention furthermore relates to a method for determining thequality of an output signal to be generated by a signal processingcircuit with respect to a reference signal, which method comprises thefollowing steps of

generating a first signal parameter as a function of time and frequencyin response to the output signal,

compressing a first signal parameter and generating a first compressedsignal parameter,

generating a second compressed signal parameter in response to thereference signal,

determining a differential signal on the basis of the compressed signalparameters, and

generating a quality signal by integrating the differential signal withrespect to time and frequency.

The method according to the invention has the characteristic that themethod furthermore comprises the following steps of

converting at least two signal parameters into a third signal parameter,and

discounting the third signal parameter after determination of thedifferential signal and before generation of the quality signal.

A first embodiment of the method according to the invention has thecharacteristic that the method comprises the following steps of

converting at least a signal parameter at a first timepoint and at afirst frequency and another signal parameter at a second timepoint andat the first frequency into a fourth signal parameter at the firstfrequency,

converting a further signal parameter at a first timepoint and at asecond frequency and another further signal parameter at a secondtimepoint and at the second frequency into a further fourth signalparameter at the second frequency, and

discounting the third signal parameter comprising the fourth signalparameter and the further fourth signal parameter before thedifferential signal is integrated with respect to time and frequency.

A second embodiment of the method according to the invention has thecharacteristic that the method comprises the following steps of

converting at least a signal parameter at a first timepoint and at afirst frequency and another signal parameter at the first timepoint andat a second frequency into the third signal parameter at the firsttimepoint, and

discounting the third signal parameter after the differential signal hasbeen integrated with respect to frequency and before the differentialsignal is integrated with respect to time.

A third embodiment of the method according to the invention has thecharacteristic that the step of generating a second compressed signalparameter in response to the reference signal comprises the followingtwo steps of

generating a second signal parameter in response to the reference signalas a function of both time and frequency, and

compressing a second signal parameter.

A fourth embodiment of the method according to the invention has thecharacteristic that the step of generating a first signal parameter inresponse to the output signal as a function of time and frequencycomprises the following two steps of

multiplying in the time domain a still further first signal to begenerated in response to the output signal by a window function, and

transforming the still further first signal to be multiplied by thewindow function to the frequency domain, which represents, afterdetermining an absolute value, a signal parameter as a function of timeand frequency.

A fifth embodiment of the method according to the invention has thecharacteristic that the step of generating a first signal parameter inresponse to the output signal as a function of time and frequencycomprises the following step of

filtering a still further first signal to be generated in response tothe output signal, which represents, after determining an absolutevalue, a signal parameter as a function of time and frequency.

A sixth embodiment of the method according to the invention has thecharacteristic that the step of generating a first signal parameter inresponse to the output signal as a function of time and frequency alsocomprises the following step of

converting a signal parameter represented by means of a time spectrumand a frequency spectrum to a signal parameter represented by means of atime spectrum and a Bark spectrum.

C. REFERENCES

▪"Modelling a Cognitive Aspect in the Measurement of the Quality ofMusic Codecs", by John G. Beerends and Jan A. Stemerdink, presented atthe 96th Convention Feb. 26-Mar. 1, 1994, Amsterdam

▪U.S. Pat. No. 4,860,360

▪EP 0 627 727

▪EP 0 417 739

▪DE 37 08 002

All the references are deemed to be incorporated by reference herein.

D. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail by reference to anexemplary embodiment shown in the figures. In the figures:

FIG. 1 shows a device according to the invention, comprising knownsignal processing arrangements, known compressing arrangements, and acombining circuit according to the invention,

FIG. 2 shows a known signal processing arrangement for use in the deviceaccording to the invention,

FIG. 3 shows a known compressing arrangement for use in the deviceaccording to the invention,

FIG. 4 shows a scaling circuit for use in the device according to theinvention,

FIG. 5 shows a combining circuit according to the invention or use inthe device according to the invention, and

FIG. 6 graphically depicts a known characteristic for time constant τ,used in time-domain smearing, as a function of frequency.

E. DETAILED DESCRIPTION

The device according to the invention shown in FIG. 1 comprises a firstsignal processing arrangement 1 having a first input 7 for receiving anoutput signal originating from a signal processing circuit such as, forexample, a coder/decoder, or codec. A first output of first signalprocessing arrangement 1 is connected via a coupling 9 to a first inputof a scaling circuit 3. The device according to the inventionfurthermore comprises a second signal processing arrangement 2 having asecond input 8 for receiving an input signal to be fed to the signalprocessing circuit such as, for example, the coder/decoder, or codec. Asecond output of second signal processing arrangement 2 is connected viaa coupling 10 to a second input of scaling circuit 3. A first output ofscaling circuit 3 is connected via a coupling 11 to a first input of afirst compressing arrangement 4, and a second output of scaling circuit3 is connected via a coupling 12 to a second input of a secondcompressing arrangement 5. A first output of first compressingarrangement 4 is connected via a coupling 13 to a first input of acombining circuit 6, and a second output of second compressingarrangement 5 is connected via a coupling 16 to a second input ofcombining circuit 6. A third output of scaling circuit 3 is connectedvia a coupling 14 to a third input of combining circuit 6, and thesecond output of second compressing arrangement 5, or coupling 16, isconnected via a coupling 15 to a fourth input of combining circuit 6which has an output 17 for generating a quality signal. The first outputof first signal processing arrangement 1 is connected via a coupling 18to a fifth input of combining circuit 6. First signal processingarrangement 1 and first compressing arrangement 4 jointly correspond toa first series circuit, and second signal processing arrangement 2 andsecond compressing arrangement 5 jointly correspond to a second seriescircuit.

The known first (or second) signal processing arrangement 1 (or 2) shownin FIG. 2 comprises a first (or second) multiplying arrangement 20 formultiplying in the time domain the output signal (or input signal) to befed to the first input 7 (or second input 8) of the first (or second)signal processing arrangement 1 (or 2) and originating from the signalprocessing circuit such as, for example, the coder/decoder, or codec, bya window function, a first (or second) transforming arrangement 21,coupled to the first (or second) multiplying arrangement 20, fortransforming the signal originating from the first (or second)multiplying arrangement 20 to the frequency domain, a first (or second)absolute-value arrangement 22 for determining the absolute value of thesignal originating from the first (or second) transforming arrangement21 for generating a first (or second) positive signal parameter as afunction of time and frequency, a first (or second) convertingarrangement 23 for converting the first (or second) positive signalparameter originating from the first (or second) absolute-valuearrangement 22 and represented by means of a time spectrum and afrequency spectrum into a first (or second) signal parameter representedby means of a time spectrum and a Bark spectrum, and a first (or second)discounting arrangement 24 for discounting a hearing function in thecase of the first (or second) signal parameter originating from thefirst (or second) converting arrangement and represented by means of atime spectrum and a Bark spectrum, which signal parameter is thentransmitted via the coupling 9 (or 10).

The known first (or second) compressing arrangement 4 (or 5) shown inFIG. 3 receives via coupling 11 (or 12) a signal parameter which is fedto a first (or second) input of a first (or second) adder 30, a first(or second) output of which is connected via a coupling 31, on the onehand, to a first (or second) input of a first (or second) multiplier 32and, on the other hand, to a first (or second) nonlinear convolutingarrangement 36 which is furthermore connected to a first (or second)compressing unit 37 for generating via coupling 13 (or 16) a first (orsecond) compressed signal parameter. First (or second) multiplier 32 hasa further first (or second) input for receiving a feed signal and has afirst (or second) output which is connected to a first (or second) inputof a first (or second) delay arrangement 34, a first (or second) outputof which is coupled to a further first (or second) input of the first(or second) adder 30.

The scaling circuit 3 shown in FIG. 4 comprises a further integratingarrangement 40, a first input of which is connected to the first inputof scaling circuit 3 and consequently to coupling 9 for receiving afirst series circuit signal (the first signal parameter represented bymeans of a time spectrum and a Bark spectrum) and a second input ofwhich is connected to the second input of scaling circuit 3 andconsequently to coupling 10 for receiving a second series circuit signal(the second signal parameter represented by means of a time spectrum anda Bark spectrum). A first output of further integrating arrangement 40for generating the integrated first series circuit signal is connectedto a first input of a comparing arrangement 41 and a second output offurther integrating arrangement 40 for generating the integrated secondseries circuit signal is connected to a second input of comparingarrangement 41. The first input of scaling circuit 3 is connected to thefirst output and, via scaling circuit 3, coupling 9 is consequentlyconnected through to coupling 11. The second input of scaling circuit 3is connected to a first input of a further scaling unit 42 and a secondoutput is connected to an output of further scaling unit 42 and, viascaling circuit 3, coupling 10 is consequently connected through tocoupling 12 via further scaling unit 42. An output of comparingarrangement 41 for generating a control signal is connected to a controlinput of further scaling unit 42. The first input of scaling circuit 3,or coupling 9 or coupling 11, is connected to a first input of a ratiodetermining arrangement 43 and the output of further scaling unit 42, orcoupling 12, is connected to a second input of ratio-determiningarrangement 43, an output of which is connected to the third output ofscaling circuit 3 and consequently to coupling 14 for generating afurther scaling signal.

The combining circuit 6 shown in FIG. 5 comprises a further comparingarrangement 50, a first input of which is connected to the first inputof combining circuit 6 for receiving the first compressed signalparameter via coupling 13 and a second input of which is connected tothe second input of combining circuit 6 for receiving the secondcompressed signal parameter via coupling 16. The first input ofcombining circuit 6 is furthermore connected to a first input of adifferential arrangement 54,56. An output of further comparingarrangement 50 for generating a scaling signal is connected via acoupling 51 to a control input of scaling arrangement 52, an input ofwhich is connected to the second input of combining circuit 6 forreceiving the second compressed signal parameter via coupling 16 and anoutput of which is connected via a coupling 53 to a second input ofdifferential arrangement 54,56 for determining a differential signal onthe basis of the mutually scaled compressed signal parameters. A thirdinput of the differential arrangement 54,56 is connected to the fourthinput of the combining circuit 6 for receiving, via coupling 15, thesecond compressed signal parameter to be received via coupling 16.Differential arrangement 54,56 comprises a differentiator 54 forgenerating a differential signal and a further absolute-valuearrangement 56 for determining the absolute value of the differentialsignal, an output of which is connected to an input of scaling unit 57,a control input of which is connected to the third input of combiningcircuit 6 for receiving the further scaling signal via coupling 14. Anoutput of scaling unit 57 is connected to an input of discountingarrangement 61, of which a control input is coupled to an output ofconverting arrangement 60. An input of converting arrangement 60 iscoupled to the fifth input of combining circuit 6 for receiving at leasttwo signal parameters and converting them into a third signal parameter.An output of discounting arrangement 61 is connected to an input of anintegrating arrangement 58,59 for integrating the scaled absolute valueof the differential signal with respect to time and frequency.Integrating arrangement 58,59 comprises a series arrangement of anintegrator 58 and a time-averaging arrangement 59, an output of which isconnected to the output 17 of combining circuit 6 for generating thequality signal.

The operation of a known device for determining the quality of theoutput signal to be generated by the signal processing circuit such as,for example, the coder/decoder, or codec, which known device is formedwithout the scaling circuit 3 shown in greater detail in FIG. 4, thecouplings 10 and 12 consequently being mutually connected through, andwhich known device is formed using a standard combining circuit 6, thethird input, shown in greater detail in FIG. 5, of differentialarrangement 54,56, and scaling unit 57, and discounting arrangement 61and converting arrangement 60 consequently being missing, is as followsand, indeed, as also described in the first reference.

The output signal of the signal processing circuit such as, for example,the coder/decoder, or codec, is fed to input 7, after which the firstsignal processing circuit 1 converts said output signal into a firstsignal parameter represented by means of a time spectrum and a Barkspectrum. This takes place by means of the first multiplying arrangement20 which multiplies the output signal represented by means of a timespectrum by a window function represented by means of a time spectrum,after which the signal thus obtained and represented by means of a timespectrum is transformed by means of first transforming arrangement 21 tothe frequency domain, for example by means of an FFT, or fast Fouriertransform, after which the absolute value of the signal thus obtainedand represented by means of a time spectrum and a frequency spectrum isdetermined by means of the first absolute-value arrangement 22, forexample by squaring, after which the signal parameter thus obtained andrepresented by means of a time spectrum and a frequency spectrum isconverted by means of first converting arrangement 23 into a signalparameter represented by means of a time spectrum and a Bark spectrum,for example by resampling on the basis of a nonlinear frequency scale,also referred to as Bark scale, which signal parameter is then adjustedby means of first discounting arrangement 24 to a hearing function, oris filtered, for example by multiplying by a characteristic representedby means of a Bark spectrum.

The first signal parameter thus obtained and represented by means of atime spectrum and a Bark spectrum is then converted by means of thefirst compressing arrangement 4 into a first compressed signal parameterrepresented by means of a time spectrum and a Bark spectrum. This takesplace by means of first adder 30, first multiplier 32 and first delayarrangement 34, the signal parameter represented by means of a timespectrum and a Bark spectrum being multiplied by a feed signalrepresented by means of a Bark spectrum such as, for example, anexponentially decreasing signal, after which the signal parameter thusobtained and represented by means of a time spectrum and a Bark spectrumis added, with a delay in time, to the signal parameter represented bymeans of a time spectrum and a Bark spectrum, after which the signalparameter thus obtained and represented by means of a time spectrum anda Bark spectrum is convoluted by means of first nonlinear convolutingarrangement 36 with a spreading function represented by means of a Barkspectrum, after which the signal parameter thus obtained and representedby means of a time spectrum and a Bark spectrum is compressed by meansof first compressing unit 37.

In a corresponding manner, the input signal of the signal processingcircuit such as, for example, the coder/decoder, or codec, is fed toinput 8, after which the second signal processing circuit 2 convertssaid input signal into a second signal parameter represented by means ofa time spectrum and a Bark spectrum, and the latter is converted bymeans of the second compressing arrangement 5 into a second compressedsignal parameter represented by means of a time spectrum and a Barkspectrum.

The first and second compressed signal parameters, respectively, arethen fed via the respective couplings 13 and 16 to combining circuit 6,it being assumed for the time being that this is a standard combiningcircuit which lacks the third input of differential arrangement 54,56,and scaling unit 57, and discounting arrangement 61 and convertingarrangement 60, all as shown in detail in FIG. 5. The two compressedsignal parameters are integrated by further comparing arrangement 50 andmutually compared, after which further comparing arrangement 50generates the scaling signal which represents, for example, the averageratio between the two compressed signal parameters. The scaling signalis fed to scaling arrangement 52 which, in response thereto, scales thesecond compressed signal parameter (that is to say, increases or reducesit as a function of the scaling signal). Obviously, scaling arrangement52 could also be used, in a manner known to the person skilled in theart, for scaling the first compressed signal parameter instead of forscaling the second compressed signal parameter and use could furthermorebe made, in a manner known to the person skilled in the art, of twoscaling arrangements for mutually scaling the two compressed signalparameters at the same time. The differential signal is derived by meansof differentiator 54 from the mutually scaled compressed signalparameters, the absolute value of which differential signal is thendetermined by means of further absolute-value arrangement 56. The signalthus obtained is integrated by means of integrator 58 with respect to aBark spectrum and is integrated by means of time averaging arrangement59 with respect to a time spectrum and generated by means of output 17as quality signal which indicates in an objective manner the quality ofthe signal processing circuit such as, for example, the coder/decoder orcodec.

The operation of the device according to the invention for determiningthe quality of the output signal to be generated by the signalprocessing circuit such as, for example, the coder/decoder, or codec,which device according to the invention is consequently formed with thescaling circuit 3 shown in detail in FIG. 4. The couplings 10 and 12 areconsequently coupled through mutually via further scaling unit, andwhich known device is formed with an expanded combining circuit 6according to the invention to which the third input of differentialarrangement 54,56 shown in greater detail in FIG. 5, and scaling unit57, and discounting arrangement 61 and converting arrangement 60 haveconsequently been added is as described above, supplemented by whatfollows.

The first series circuit signal (the first signal parameter representedby means of a time spectrum and a Bark spectrum) to be received viacoupling 9 and the first input of scaling circuit 3 is fed to the firstinput of further integrating arrangement 40 and the second seriescircuit signal (the second signal parameter represented by means of atime spectrum and a Bark spectrum) to be received via the coupling 10and the second input of scaling circuit 3 is fed to the second input offurther integrating arrangement 40, which integrates the two seriescircuit signals with respect to frequency, after which the integratedfirst series circuit signal is fed via the first output of furtherintegrating arrangement 40 to the first input of comparing arrangement41 and the integrated second series circuit signal is fed via the secondoutput of further integrating arrangement 40 to the second input ofcomparing arrangement 41. The latter compares the two integrated seriescircuit signals and generates, in response thereto, the control signalwhich is fed to the control input of further scaling unit 42. The latterscales the second series circuit signal (the second signal parameterrepresented by means of a time spectrum and a Bark spectrum) to bereceived via coupling 10 and the second input of scaling circuit 3 as afunction of said control signal (that is to say increases or reduces theamplitude of the second series circuit signal) and generates the thusscaled second series circuit signal via the output of further scalingunit 42 to the second output of scaling circuit 3, while the first inputof scaling arrangement 3 is connected through in this example in adirect manner to the first output of scaling circuit 3. In this example,the first series circuit signal and the scaled second series circuitsignal, respectively are passed via scaling circuit 3 to firstcompressing arrangement 4 and second compressing arrangement 5,respectively.

As a result of this further scaling, a good correlation is obtainedbetween the objective quality signal to be assessed by means of thedevice according to the invention and a subjective quality signal to beassessed by human observers. This all is based, inter alia, on theinsight that the poor correlation between objective quality signals, tobe assessed by means of known devices, and subjective quality signals,to be assessed by human observers, is the consequence, inter alia, ofthe fact that certain distortions are found to be more objectionable byhuman observers than other distortions, which poor correlation isimproved by using the two compressing arrangements, and is furthermorebased, inter alia, on the insight that, as a result of using scalingcircuit 3, the two compressing arrangements 4 and 5 function better withrespect to one another, which further improves the correlation. So, theproblem of the poor correlation can be solved by an improved functioningof the two compressing arrangements 4 and 5 with respect to one anotheras a result of using scaling circuit 3.

As a result of the fact that the first input of scaling circuit 3, orcoupling 9 or coupling 11, is connected to the first input ofratio-determining arrangement 43 and the output of further scaling unit42, or coupling 12, is connected to the second input ofratio-determining arrangement 43, ratio-determining arrangement 43 iscapable of assessing a mutual ratio of the first series circuit signaland the scaled second series circuit signal and of generating a furtherscaling signal as a function thereof by means of the output ofratio-determining arrangement 43, which further scaling signal is fedvia the third output of scaling circuit 3 and consequently via coupling14 to the third input of combining circuit 6. The further scaling signalis fed in combining circuit 6 to scaling unit 57 which scales, as afunction of the further scaling signal, the absolute value of thedifferential signal originating from the differential arrangement 54,56(that is to say increases or reduces the amplitude of said absolutevalue). As a consequence thereof, the already improved correlation isimproved further as a result of the fact an (amplitude) difference stillpresent between the first series circuit signal and the scaled secondseries circuit signal in the combining circuit is discounted andintegrating arrangement 58,59 functions better as a result.

A further improvement of the correlation is obtained if differentiator54 (or further absolute-value arrangement 56) is provided with a furtheradjusting arrangement, not shown in the figures, for example in the formof a subtracting circuit which reduces somewhat the amplitude of thedifferential signal. Preferably, the amplitude of the differentialsignal is reduced as a function of a series circuit signal, just as inthis example it is reduced as a function of the scaled and compressedsecond signal parameter originating from second compressing arrangement5, as a result of which integrating arrangement 58,59 functions stillbetter. As a result, the correlation, which is already very good isimproved still further.

Another further improvement of the correlation is obtained if combiningcircuit 6 is provided with discounting arrangement 61, of which acontrol input is coupled to the first and/or second series circuit viaconverting arrangement 60. In case of converting arrangement 60 beingcoupled to the first series circuit, the first signal parametersoriginating from first signal processing circuit 1 are supplied to theinput of converting arrangement 60. These first signal parameters arerepresented by means of a time spectrum and a frequency spectrum (inparticular a Bark spectrum). Table 1 shows sixteen first signalparameters X, each one at one out of four timepoints t₁ -t₄ and at oneout of four frequencies f₁ -f₄ :

                  TABLE 1                                                         ______________________________________                                               t.sub.1                                                                            t.sub.2      t.sub.3                                                                              t.sub.r                                       ______________________________________                                        f.sub.1  X.sub.t1.f1                                                                          X.sub.t2,f1  X.sub.t3,f1                                                                        X.sub.t4,f1                                 f.sub.2  X.sub.t1,f2                                                                          X.sub.t2,f2  X.sub.t3,f2                                                                        X.sub.t4,f2                                 f.sub.3  X.sub.t1,f3                                                                          X.sub.t2,f3  X.sub.t3,f3                                                                        X.sub.t4,f3                                 f.sub.4  X.sub.t1,f4                                                                          X.sub.t2,f4  X.sub.t3,f4                                                                        X.sub.t4,f4                                 ______________________________________                                    

According to a first embodiment (discounting arrangement 61 is situatedbetween differential arrangement 54,56 and integrator 58). Convertingarrangement 60 converts for example the four signal parametersX_(t1),f1, X_(t2),f1, X_(t3),f1, X_(t4),f1 into a fourth signalparameter Y_(f1), and converts the four signal parameters X_(t1),f2,X_(t2),f2, X_(t3),f2, X_(t4),f2 into a further fourth signal parameterY_(f2), and converts the four signal parameters X_(t1),f3, X_(t2),f3,X_(t3),f3, X_(t4),f3 into a still further fourth signal parameterY_(f3), and converts the four signal parameters X_(t1),f4, X_(t2),f4,X_(t3),f4, X_(t4),f4 into a yet still further fourth signal parameterY_(f4). This converting is, for example, realized by calculating anaverage value of each of the four signal parameters, and then taking anabsolute difference between the last one of each four signal parametersand the corresponding average value. The four fourth signal parametersare supplied to the control input of discounting arrangement 61. At itsinput discounting arrangement 61 receives the differential signalcomprising four parameters Z_(t4),f1, Z_(t4),f2, Z_(t4),f3, Z_(t4),f3,Z_(t4),f4 and generates at its output these four signal parameters, eachone being divided by the corresponding fourth signal parameter:Z_(t4),f1 /Y_(f1), Z_(t4),f2 /Y_(f2), Z_(t4),f3 /Y_(f3), Z_(t) ₄,f4/Y_(f4).

According to a second embodiment (discounting arrangement 61 should besituated between integrator 58 and time-averaging arrangement 59),converting arrangement 60 converts, for example, the four signalparameters X_(t4),f1, X_(t4),f2, X_(t4),f3, X_(t4),f4 into a thirdsignal parameter W_(t4). This converting is, for example, realized bycalculating the average value of these four signal parameters, thencalculating the difference between each one of these four signalparameters and the average value, squaring each calculated difference,summing the squared calculated differences and rooting this sum, therooted sum being equal to the third signal parameter W_(t4). This thirdsignal parameter is supplied to the control input of discountingarrangement 61. At its input, discounting arrangement 61 receives asignal V_(t4) coming from integrator 58, and generates at its outputthis signal, being divided by the third signal parameter: V_(t4)/t_(t4).

According to a third embodiment (discounting arrangement 61 should besituated between integrator 58 and time-averaging arrangement 59),converting arrangement 60 converts, for example, the four signalparameters X_(t4),f1, X_(t4),f2, X_(t4),f3, X_(t4),f4, into a thirdsignal parameter W_(t4). This converting is, for example, realized bycalculating the average value of Y_(f1), Y_(f2), Y_(f3), Y_(f4), thencalculating the difference between each one of these four signalparameters X_(t4),f1, X_(t4),f2, X_(t4),f3, X_(t4),f4 and the averagevalue, squaring each calculated difference, summing the squaredcalculated differences and rooting this sum, the rooted sum being equalto the third signal parameter W_(t4). This third signal parameter issupplied to the control input of discounting arrangement 61. At itsinput, discounting arrangement 61 receives a signal V_(t4) coming fromintegrator 58, and generates at its output this signal, being divided bythe third signal parameter: V_(t4) /W_(t4).

As a result of providing the device with converting arrangement 60 anddiscounting arrangement 61, the complexity of the reference signal oroutput signal can be used to adjust the quality signal. Due to theconverting and discounting, a good correlation is obtained between theobjective quality signal, to be assessed by means of said device, and asubjective quality signal, to be assessed by human observers. Theinvention is based, inter alia, on the insight that the poor correlationbetween objective quality signals, to be assessed by means of knowndevices, and subjective quality signals, to be assessed by humanobservers, is the consequence, inter alia, of the fact that certaindistortions are found to be more objectionable by human observers thanother distortions, which poor correlation is improved by using the twocompressing arrangements, and is furthermore based, inter alia, on theinsight that distortions in a less complex signal are found to be moreobjectionable than distortions in a more complex signal.

Usually discounting arrangement 61 and converting arrangement 60 will besituated inside combining circuit 6. However, converting arrangement 60could, for example, also be placed inside one of the series circuits.Although in FIG. 1 the fifth input of combining circuit 6 is coupled tothe first series circuit (the first output of first signal processingarrangement 1), this fifth input could also be coupled to the secondseries circuit (for example, the second output of second signalprocessing circuit 2). Recent proof shows that this will improve thecorrelation even more.

The components shown in FIG. 2 of first signal processing arrangement 1are described, as stated earlier, adequately and in a manner known tothe person skilled in the art. In that regard and for further details,see John G. Beerends and Jan A. Stemerdink, "A Perceptual Audio QualityMeasure Based on a Psychoacoustic Sound Representation", Journal of theAudio Engineering Society, Vol. 40, No. 12, December 1992, pages 963-978(hereinafter the "Beerends" et al paper"). Specifically, as to signalprocessing arrangement, a digital output signal which originates fromthe signal processing circuit such as, for example, the coder/decoder,or codec, and which is, for example, discrete both in time and inamplitude is multiplied by means of first multiplying arrangement 20 bya window function such as, for example, a so-called cosine squarefunction represented by means of a time spectrum, after which the signalthus obtained and represented by means of a time spectrum is transformedby means of first transforming arrangement 21 to the frequency domain,for example by an FFT, or fast Fourier transform, after which theabsolute value of the signal thus obtained and represented by means of atime spectrum and a frequency spectrum is determined by means of thefirst absolute-value arrangement 22, for example by squaring. Finally, apower density function per time/frequency unit is thus obtained. Analternative way of obtaining said signal is to use a subband filteringarrangement for filtering the digital output signal, which subbandfiltering arrangement generates, after determining an absolute value, asignal parameter as a function of time and frequency in the form of thepower density function per time/frequency unit. First convertingarrangement 23 converts the power density function per time/frequencyunit, for example, by resampling on the basis of a nonlinear frequencyscale, also referred to as Bark scale, into a power density function pertime/Bark unit, which conversion is known in the art and describedcomprehensively in Appendix A of the Beerends et al paper, and firstdiscounting arrangement 24 multiplies said power density function pertime/Bark unit, for example by a characteristic, represented by means ofa Bark spectrum, for performing an adjustment on a hearing function.

The components, shown in FIG. 3, of first compressing arrangement 4 are,as stated earlier, known in the art and described adequately and in amanner known to the person skilled, in the art in the Beerends et alpaper. Specifically with respect to the first compression arrangement,the density function per time/Bark unit adjusted to a hearing functionis multiplied by means of multiplier 32 by an exponentially decreasingsignal such as, for example, exp{-T/τ(z)}. Here T is equal to 50% of thelength of the window function and consequently represents half of acertain time interval, after which certain time interval firstmultiplying arrangement 20 always multiplies the output signal by awindow function represented by means of a time spectrum (for example,50% of 40 msec is 20 msec). In this expression, τ(z) is a characteristicwhich is represented by means of the Bark spectrum and is shown indetail in FIG. 6 of the Beerends et al paper. First delay arrangement 34delays the product of this multiplication by a delay time of length T,or half of the certain time interval. First nonlinear convolutionarrangement 36 convolves the signal supplied by a spreading functionrepresented by means of a Bark spectrum, or spreads a power densityfunction represented per time/Bark unit along a Bark scale, which isknown in the art and described comprehensively in Appendix B of theBeerends et al paper. First compressing unit 37 compresses the signalsupplied in the form of a power density function represented pertime/Bark unit with a function which, for example, raises the powerdensity function represented per time/Bark unit to the power α, where0<α<1.

The components, shown in FIG. 4, of scaling circuit 3 can be formed in amanner known to the person skilled in the art. Further integratingarrangement 40 comprises, for example, two separate integrators whichseparately integrate the two series circuit signals supplied by means ofa Bark spectrum, after which comparing arrangement 41 in the form of,for example, a divider, divides the two integrated signals by oneanother and feeds the division result or the inverse division result asthe control signal to further scaling unit 42 which, in the form of, forexample, a multiplier or a divider, multiplies or divides the secondseries circuit signal by the division result or the inverse divisionresult in order to make the two series circuit signals, viewed onaverage, of equal size. Ratio-determining arrangement 43 receives thefirst and the scaled second series circuit signal in the form ofcompressed, spread power density functions represented per time/Barkunit and divides them by one another to generate the further scalingsignal in the form of the division result represented per time/Bark unitor the inverse thereof, depending on whether scaling unit 57 isconstructed as multiplier or as divider.

The components, shown in FIG. 5, of first combining circuit 6 are, asstated earlier, well known in the art and described adequately and in amanner known to the person skilled in the art in the Beerends et alpaper, with the exception of the component 57 and a portion of component54. Further comparing arrangement 50 comprises, for example, twoseparate integrators which separately integrate the two series circuitsignals supplied over, for example, three separate portions of a Barkspectrum and comprises, for example, a divider which divides the twointegrated signals by one another per portion of the Bark spectrum andfeeds the division result or the inverse division result as the scalingsignal to scaling arrangement 52 which, in the form of, for example, amultiplier or a divider, multiplies or divides the respective seriescircuit signal by the division result or the inverse division result inorder to make the two series circuit signals, viewed on average, ofequal size per portion of the Bark spectrum. Since the above is wellknown in the art, for further details the reader is directed to AppendixF of the Beerends et al paper. Differentiator 54 determines thedifference between the two mutually scaled series circuit signals.According to the invention, if the difference is negative, thedifference can then be augmented by a constant value and, if thedifference is positive, the difference can be reduced by a constantvalue, for example, by detecting whether it is less or greater than thevalue zero and then adding or subtracting the constant value. It is,however, also possible first to determine the absolute value of thedifference by means of further absolute-value arrangement 56 and then todeduct the constant value from said absolute value, in which connectiona negative final result must obviously not be permitted to be obtained.In this last case, absolute-value arrangement 56 should be provided witha subtracting circuit. Furthermore, it is possible, according to theinvention, to discount from the difference a (portion of a) seriescircuit signal in a similar manner instead of the constant value ortogether with the constant value. Integrator 58 integrates the signaloriginating from scaling unit 57 with respect to a Bark spectrum andtime-averaging arrangement 59 integrates the signal thus obtained withrespect to a time spectrum, as a result of which the quality signal isobtained which has a value which is the smaller, the higher the qualityof the signal processing circuit is.

As already described earlier, the correlation between the objectivequality signal, to be assessed by means of the device according to theinvention, and a subjective quality signal, to be assessed by humanobservers, is improved by several factors which can be viewed separatelyfrom one another:

the use of discounting arrangement 61 and converting arrangement 60,discounting arrangement 61 being situated between differentialarrangement 54,56 and integrating arrangement 58,59,

the use of discounting arrangement 61 and converting arrangement 60,discounting arrangement 61 being situated between integrator 58 andtime-averaging arrangement 59,

the use of the scaling circuit 3 without making use of theratio-determining arrangement 43 and scaling unit 57,

the use of the scaling circuit 3 with use being made of ratiodetermining arrangement 43 and scaling unit 57,

the use of differential arrangement 54,56 which is provided with thethird input for receiving a signal having a certain value, which signalshould be deducted from the difference to be determined originally, and

the use of differential arrangement 54,56 which is provided with thethird input for receiving a further signal derived from a series circuitsignal having a further certain value, which further signal should bededucted from the difference to be determined originally.

The best correlation is obtained by simultaneous use of several of theabove factors.

The widest meaning should be reserved for the term signal processingcircuit, in which connection, for example, all kinds of audio and/orvideo equipment can be considered. Thus, the signal processing circuitcould be a codec, in which case the input signal is the reference signalwith respect to which the quality of the output signal should bedetermined. The signal processing circuit could also be an equalizer, inwhich connection the quality of the output signal should be determinedwith respect to a reference signal which is calculated on the basis ofan already existing virtually ideal equalizer or is simply calculated.The signal processing circuit could even be a loudspeaker, in which casea smooth output signal could be used as reference signal, with respectto which the quality of a sound output signal is then determined(scaling already takes place automatically in the device according tothe invention). The signal processing circuit could furthermore be aloudspeaker computer model which is used to design loudspeakers on thebasis of values to be set in the loudspeaker computer model, in whichcase a low-volume output signal of said loudspeaker computer modelserves as the reference signal and a high-volume output signal of saidloudspeaker computer model then serves as the output signal of thesignal processing circuit.

In the case of a calculated reference signal, the second signalprocessing arrangement of the second series circuit could be omitted asa result of the fact that the operations to be performed by the secondsignal processing arrangement can be discounted in calculating thereference signal. In that case, the reference signal could be suppliedto converting arrangement 60 as well.

What is claimed is:
 1. A method for determining audio quality of anoutput signal generated by a signal processing circuit with respect to areference signal, the method comprising the steps of:generating a firstsignal parameter as a function of time and frequency in response to theoutput signal; compressing said first signal parameter so as to yield afirst compressed signal parameter; generating a second compressed signalparameter in response to the reference signal; determining a differencesignal in response to the first and second compressed signal parameters;generating a quality signal in response to the difference signal,through integration with respect to frequency and time; converting atleast two further signal parameters into a third signal parameter, saidat least two further signal parameters being derived from one of saidfirst signal parameter and a second signal parameter, one of said atleast two further signal parameters being at a first time-point and at afirst frequency and an other of said at least two further signalparameters being at a second time-point and at a second frequency,wherein either the first and second time-points or the first and secondfrequencies are different from each other; and discounting the thirdsignal parameter during the step of generating the quality signal so asto yield the discounted third signal parameter.
 2. The method accordingto claim 1 further comprising the steps of:converting at least a signalparameter at the first time-point and at the first frequency and anothersignal parameter at the second time-point and at the first frequencyinto a fourth signal parameter at the first frequency; converting afurther signal parameter at the first time-point and at the secondfrequency and another further signal parameter at the second time-pointand at the second frequency into a further fourth signal parameter atthe second frequency; and discounting the third signal parametercomprising the fourth signal parameter and the further fourth signalparameter before the difference signal is integrated with respect totime and frequency.
 3. The method according to claim 1 furthercomprising the steps of:converting at least a signal parameter at thefirst time-point and at the first frequency and an other signalparameter at the first time-point and at the second frequency into thethird signal parameter at the first time-point; and discounting thethird signal parameter after the difference signal has been integratedwith respect to frequency but before the difference signal is integratedwith respect to time.
 4. The method according to claim 1 furthercomprising the step of generating a second compressed signal parameterin response to the reference signal, wherein the second compressedsignal parameter compressing step comprises the steps of:generating saidsecond signal parameter in response to the reference signal as afunction of both time and frequency; and compressing the second signalparameter so as to yield the compressed second signal parameter.
 5. Themethod according to claim 1 further comprising the step of generatingthe first signal parameter in response to the output signal as afunction of time and frequency, wherein the first signal parametergenerating step comprises the steps of:multiplying, in a time domain, astill further first signal, generated in response to the output signal,by a window function; and transforming the still further first signalmultiplied by the window function to the frequency domain, whichrepresents, after determining an absolute value thereof, a signalparameter as a function of time and frequency.
 6. The method accordingto claim 5 wherein the step of generating the first signal parameter inresponse to the output signal as a function of time and frequencyfurther comprises the step of converting a signal parameter representedthrough a time spectrum and a frequency spectrum to a signal parameterrepresented through a time spectrum and a Bark spectrum.
 7. The methodaccording to claim 1 further comprising the step of generating the firstsignal parameter in response to the output signal as a function of timeand frequency, wherein the first signal parameter generating stepcomprises the step of filtering a still further first signal, generatedin response to the output signal, which represents, after determining anabsolute value thereof, a signal parameter as a function of time andfrequency.
 8. A device for determining audio quality of an output signalgenerated by a signal processing circuit with respect to a referencesignal, the device having a first series circuit having a first inputfor receiving the output signal, a second series circuit having a secondinput for receiving the reference signal, and a combining circuit,coupled to a first output of the first series circuit and to a secondoutput of the second series circuit, for generating a quality signal,A)wherein the first series circuit comprises:A1) a first signal processingarrangement, coupled to the first input, for generating a first signalparameter as a function of time and frequency; and A2) a firstcompressing arrangement, coupled to the first signal processingarrangement, for compressing a first signal parameter and for generatinga first compressed signal parameter; and B) wherein the second seriescircuit comprises:B1) a second compressing arrangement, coupled to thesecond input, for generating a second compressed signal parameter; andC) wherein the combining circuit comprises:C1) a differentialarrangement, coupled to the first and second compressing arrangements,for determining a difference signal on the basis of the first and secondcompressed signal parameters; and C2) an integrating arrangement, forgenerating the quality signal in response to the difference signal,through integration with respect to frequency and time; D) a convertingarrangement, responsive to at least one of the first and second signalparameters for receiving at least two signal parameters and convertingsaid at least two signal parameters into a third signal parameter, andhaving an output coupled to an input of a discounting arrangement, oneof said at least two signal parameters being at a first time-point andat a first frequency and an other of said at least two parameters beingat a second time-point and at a second frequency, wherein either thefirst or second time-points or the first and second frequencies aredifferent from each other; andwherein the combining circuit furthercomprises C3) the discounting arrangement for discounting the thirdsignal parameter during the generation of the quality signal in theintegrating arrangement.
 9. The device according to claim 8 wherein theconverting arrangement converts at least a signal parameter at the firsttime-point and at the first frequency and another signal parameter atthe second time-point and at the first frequency into a fourth signalparameter at the first frequency and converts a further signal parameterat a first time-point and at the second frequency and another furthersignal parameter at the second time-point and at the second frequencyinto a further fourth signal parameter at the second frequency, whereinthe discounting arrangement is situated between the differentialarrangement and the integrating arrangement, and the third signalparameter comprises the fourth signal parameter and the further fourthsignal parameter.
 10. The device according to claim 8 wherein theconverting arrangement converts at least a signal parameter at the firsttime-point and at the first frequency and another signal parameter atthe first time-point and at the second frequency into the third signalparameter at the first time-point, wherein the discounting arrangementis situated inside the integrating arrangement and discounts the thirdsignal parameter after the difference signal is integrated with respectto frequency but before the difference signal is integrated with respectto time.
 11. The device according to claim 8 wherein the second seriescircuit further comprises a second signal processing arrangement,coupled to the second input, for generating a second signal parameter asa function of both time and frequency, the second signal compressingarrangement being coupled to the second signal processing arrangement soas to compress the second signal parameter.
 12. The device according toclaim 8 wherein the first signal processing arrangement comprises:amultiplying arrangement for multiplying, in a time domain, a signal fedto an input of the first signal processing arrangement by a windowfunction; and a transforming arrangement, coupled to the multiplyingarrangement, for transforming a signal originating from the multiplyingarrangement to the frequency domain, wherein the transformingarrangement generates, after determining an absolute value, a signalparameter as a function of time and frequency.
 13. The device accordingto claim 12 wherein the first signal processing arrangement furthercomprises a converting arrangement for converting a signal parameterrepresented through a time spectrum and a frequency spectrum into asignal parameter represented through a time spectrum and a Barkspectrum.
 14. The device according to claim 8 wherein the first signalprocessing arrangement further comprises a subband filter arrangementfor filtering the signal fed to the input of the first signal processingarrangement, wherein the subband filtering arrangement generates, afterdetermining the absolute value, a signal parameter as a function of timeand frequency.